1. Field of the Invention
The present invention relates to an image processing method and apparatus in which a RIP process is performed on original image data to obtain raster data, and a smoothing process is performed through oversampling the obtained raster data to generate output image data.
2. Description of the Related Art
Image forming methods for forming a color image using an inkjet recording device are proposed. In the method, a color image is formed through the steps of: creating original image data including graphics, such as characters, illustrations, and the like, using a computer; outputting the original image data in PDL format (page description language) from the computer; performing a RIP process on the original image data in PDL format to create raster data of C (cyan), M (magenta), Y (yellow), and K (black) components; and forming the color image using an inkjet recording device based on the obtained raster data.
Here, when forming a character “A” based on the raster data created in the manner as described above, the edge portion of the character becomes irregular depending on the size of the dots (resolution) as shown in FIG. 8A.
Consequently, for inkjet recording devices having capabilities of multi-gradation representation, methods for obscuring the aforementioned irregularity are proposed as described for example, in Japanese Unexamined Patent Publication Nos. 11 (1999)-69163, 10 (1998)-336454, 5 (1993)-233792, 9 (1997)-149242, and 2001-105663. In such methods, for example, a smoothing process is performed, using an oversampling method or the like, on the raster data to generate intermediate density data, thereby obscuring the irregularity. The referent of “oversampling method” as used herein means a method in which high resolution raster data are divided into pixel groups, each pixel group including n×n pixels, and output image data are generated with the average density of each pixel group as the density of each pixel in output image.
More specifically, for example, in an inkjet recording device having capabilities of eight-gradation (8 level) representation per pixel by varying the number of ink drops to be discharged for each pixel in the range from 0 to 7, the irregularity on the edge portion of a character may be obscured in the following manner. That is, calculating the density levels using the oversampling method described above, and generating pixel values of eight gradations based on the density levels, and forming dots by varying the number of ink drops in eight steps based on the pixel values of eight gradations, and thereby arranging intermediate density dots on the edge portion of the character (FIG. 8B).
Here, for example, when generating output image data to be inputted to an inkjet recording device with an output resolution of 300×300 dpi and an output gradation value of eight by performing the oversampling described above, it has been customary to perform a RIP process on the original image data to obtain raster data with a resolution of, for example, 900×900 dpi, and to perform oversampling by dividing the raster data with the resolution of 900×900 dpi into pixel groups of 3×3 pixels to generate output image data. Here each pixel group corresponds to each pixel in output image.
Even if the number of ink drops is varied in eight steps, however, there may be a case, depending on the type of printing paper, in which the density of the dots is not represented in eight steps due to ink sheering, ink holding capacity, or ink bleeding of the printing paper. Thus, for example, generation of raster data that enable the density representation of dots in eight steps for printing paper that support the density representation of dots only in five steps (including white dots) results in that some of the data are wasted.
More specifically, in a case where the printing paper may support the density representation of dots only in five steps (including white dots), and output image data with a resolution of 300×300 dpi are to be obtained, the amount of data required is 300×300×4 (number of drops)=360000, and not more. The referent of “amount of data” as used herein means the number of gradations per unit area. Here, the amount of data is expressed in the number of gradations per square inch as an example.
That is, the RIP process needs to be performed only to obtain the raster data of two gradations with a resolution of 600×600 dpi. Thus, performance of the RIP process to obtain the raster data with a resolution of 900×900 dpi as in the conventional manner causes the RIP process time unnecessarily long. Further, the data to be processed becomes voluminous, and subsequent data processing for the raster data is also caused unnecessarily long.